1. Field of the Invention
The present invention relates to radar detection circuits and more particularly to RF pulse detection circuits. The invention can be used to detect interferometric RF signals for high resolution holographic radar, rangefinding radar, motion sensing radar, and reflectometer radar.
2. Description of Related Art
High bandwidth sample-hold circuits can be used to receive radar signals for high resolution imaging, ranging and motion sensing applications. Optimally, the sample aperture is set to match the width of the pulse being sampled. When the RF signal is a short burst of sinusoids, the sample aperture is set to one-half of an RF cycle. Ultra-wideband (UWB) emissions are defined by the FCC as having greater than 500 MHz bandwidth.
A stringent requirement for UWB radar sampling is sample timing must coincide with the expected temporal location of an echo, otherwise the sampler would miss the echo. A timing error corresponding to ½ of an RF cycle could result in no reception. Low timing jitter is also extremely important. Excessive timing jitter in pulse-averaging UWB radar can have significantly reduced sensitivity, while a non-averaging radar can exhibit a significant number of misses. UWB samplers and mixer-integrators (i.e., correlators) are phase and frequency sensitive.
Most UWB radar signals contain multiple RF lobes due to differentiation in the antenna and pulse circuitry, or due to emissions produced by short burst transmit oscillators, generally with bursts of less than 2 ns wide. At least one full RF cycle is almost always involved in UWB reception. If the sampling aperture were set to one full RF cycle, a sampler or a correlator would integrate the RF signal across the sampling aperture and produce zero output. Clearly, it is necessary to sample a half cycle in these UWB applications. In burst mode, successive repetitions of ½ cycle sampling can occur across the burst
When RF sinusoids are sampled using a UWB sampler, the sampled output is mixed-down, aliased or down-converted. In tank-level radar, a 6 GHz RF burst signal is can be down-converted to a 6 kHz expanded time burst signal by the sampler in concert with a stroboscopic timing system. Down-conversion allows signal processing to occur at greatly reduced bandwidth for reduced cost, reduced power consumption and improved accuracy.
In 1993, an averaging UWB sampler was disclosed in U.S. Patent, “Ultra-wideband Receiver,” by Thomas McEwan, the present inventor. A capacitor is connected to an RF input, e.g., an antenna, and to one end of a diode. The other end of the diode is connected to a narrow gate pulse source. The combination of the gate pulse and RF signal produces conduction in the diode and the capacitor is charged in proportion to the RF signal during the gate pulse. By using a large capacitor, a large number of conduction cycles are required to produce a quiescent voltage on the capacitor. A large number of pulses are thereby integrated on the capacitor and UWB detection and down-conversion occurs at the capacitor connected directly to the antenna. This UWB sampler is extremely simple and highly sensitive. It is directed to the reception of wideband and UWB RF signals using narrow aperture gate pulses that are matched to the UWB input signal. Gate pulse width is generally set to ½ of an RF cycle in width. Gate pulse width cannot be set to one RF cycle or to a large number of RF cycles since the integrated, sampled average would be zero, or near zero, due to the fact that a received RF cycle must have a zero average in order to propagate through free space.
While aliasing can be advantageous, limitations occur in systems where timing jitter or RF oscillator phase noise is excessive. In these cases, it would be preferable to have a phase-independent sampler. Aliasing can also be a severe detriment in range-gated interferometric radar, e.g., holographic radar where a reference wave is employed. Undesired aliasing and desired interferometric patterns can be of the same order and thus indistinguishable. A non-aliasing magnitude-only sampler is needed. A range-gated interferometric radar is disclosed in copending U.S. patent application Ser. No. 12/380,324, “Range Gated Holographic Radar,” by the present inventor, Thomas E. McEwan.